1. Field of the Invention
The present invention relates to a driving circuit of an in-plane switching (IPS) mode liquid crystal display (LCD) device, and more particularly, to a common voltage driving circuit of an IPS mode LCD device.
2. Discussion of the Related Art
A liquid crystal display (LCD) device changes optical anisotropy as a result of an electric field applied to liquid crystal having both the fluidity of liquid and the optical characteristics of crystal. Recently, the LCD device has been widely utilized because of its advantageous characteristics, such as low power consumption, thin profile, high resolution, and low size to weight ratio, as compared with a conventional cathode ray tube (CRT).
The LCD device includes an LCD panel for displaying images, and a driving circuit part for supplying driving signals to the LCD panel. Also, the LCD panel includes first and second substrates bonded to each other with a predetermined gap therebetween. A liquid crystal layer is injected between the first and second substrates.
The first substrate, known as a thin film transistor array substrate, includes a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions arranged in a matrix-type configuration, and a plurality of thin film transistors (TFTs) for switching in response to a signal on the gate line for transmission of a signal on the data line to the pixel electrode. The second substrate, known as a color filter array substrate, includes a black matrix layer for shielding light from areas other than the pixel regions, an R/G/B color filter layer for displaying various colors, and a common electrode for implementing the images. In addition, the predetermined gap is maintained between the first and second substrates by spacers. The first and second substrates are bonded to each other by a sealant having an injection inlet, through which the liquid crystal material is injected between the first and second substrates.
FIG. 1 illustrates a block diagram of a driving circuit part in an LCD device according to the related art. As shown in FIG. 1, the related art LCD device includes an LCD panel 1, a driving circuit part 2, and a backlight 8. The LCD panel 1 is formed with pixel regions in a matrix-type configuration with gate lines G and data lines D arranged perpendicular with respect to each other. The driving circuit part 2 supplies driving signals and data signals to the LCD panel 1. The backlight 8 supplies constant light to the LCD panel 1.
The driving circuit part 2 includes a data driver 1b, a gate driver 1a, a timing controller 3, a power supply part 4, a gamma reference voltage part 5, a DC/DC converter 6, and an inverter 9. The data driver 1b inputs a data signal to each data line D of the LCD panel 1. The gate driver 1a supplies a gate driving pulse to each gate line G of the LCD panel 1. The timing controller 3 receives display data R/G/B, vertical and horizontal synchronous signals Vsync and Hsync, a clock signal DCLK and a control signal DTEN from a driving system 7, and formats and outputs the display data, the clock signal DCLK and the control signal DTEN at a timing suitable for restoring a picture image by the gate driver 1a and the data driver 1b of the LCD panel 1. The power supply part 4 supplies voltages to the LCD panel 1 and other components. The gamma reference voltage part 5 receives a voltage from the power supply part 4 to provide a reference voltage required when digital data from the data driver 1b is converted to analog data. The DC/DC converter 6 outputs a constant voltage VDD, a gate high voltage (gate turn-on voltage) VGH, a gate low voltage (gate turn-off voltage) VGL, a gamma reference voltage Vref, and a common voltage Vcom for the LCD panel 1 by using a voltage output from the power supply part 4. The inverter 9 serves to drives the backlight 8. Control signals supplied to the gate driver 1a from the timing controller 3 are GSC (Gate Shift Clock), GSP (Gate Shift Pulse) and GOE (Gate Output Enable), and control signals supplied to the data driver 1b from the timing controller 3 are SSC (Source Shift Clock), SSP (Source Shift Pulse), SOE (Source Output Enable), POL (Polarity signal) and REV (Reverse signal).
An operation of the driving circuit part 2 of the related art LCD device is described as follows. As mentioned above, the timing controller 3 receives the display data R/G/B, the vertical and horizontal synchronous signals Vsync and Hsync, the clock signal DCLK, and the control signal DTEN from the driving system (PC) 7, and provides the display data, the clock signal DCLK and the control signal DTEN at the timing suitable for restoring the picture image to the gate driver 1a and the data driver 1b of the LCD panel 1. The gate driver 1a supplies the gate driving pulse to each gate line G of the LCD panel 1, and the data driver 1b synchronously inputs the data signal to each data line D of the LCD panel 1, thereby displaying the input video signal.
The LCD device has various modes according to the properties of liquid crystal and pattern structure. Specifically, LCD devices are categorized into a twisted nematic (TN) mode for controlling liquid crystal director by applying a voltage after arrangement of liquid crystal director twisted at 90°, a multi-domain mode for obtaining a wide viewing angle by dividing one pixel into several domains, an optically compensated birefringence (OCB) mode for compensating a phase change of light according to progressing direction of light by forming a compensation film on an outer surface of a substrate, an in-plane switching (IPS) mode for forming an electric field parallel to two substrates by forming two electrodes on any one substrate, and a vertical alignment (VA) mode for arranging a longitudinal (major) axis of liquid crystal molecule vertical to a plane of an alignment layer by using negative type liquid crystal and vertical alignment layer. Among them, the IPS mode LCD device generally includes a color filter substrate and a thin film transistor array substrate facing each other, and a liquid crystal layer formed between the two substrates. The color filter substrate of the IPS mode LCD device includes a black matrix layer for preventing light leakage, and an R/G/B color filter layer for realizing various colors on the black matrix layer. The thin film transistor array substrate of the IPS mode LCD device includes gate and data lines to define a unit pixel region, a switching device formed at a crossing point of the gate and data lines, and common and pixel electrodes alternately for generating an electric field across the liquid crystal.
A description of a related art IPS mode LCD device and a method for fabricating the will be made in reference to the accompanying drawings. FIG. 2 illustrates a plane view of a unit pixel in the related art IPS mode LCD device. FIG. 3 illustrates a voltage distribution of the IPS mode LCD device along line I-I′ of FIG. 2. FIG. 4A and FIG. 4B illustrate plane views of the IPS mode LCD device when a voltage is turned on/off.
FIG. 2 shows a part of a thin film transistor array substrate of the related art IPS mode LCD device. The thin film transistor array substrate includes a gate line 12, a data line 15, a thin film transistor TFT, a common line 25, a plurality of common electrodes 24, a plurality of pixel electrodes 17, and a capacitor electrode 26. Herein, the gate line 12 is formed in one direction on the thin film transistor array substrate, and the data line 15 is formed perpendicular to the gate line 12 to define a pixel region. The thin film transistor TFT is formed adjacent to a crossing portion of the gate and data lines 12 and 15. The common line 25 is then formed in parallel with the gate line 12 within the pixel region. The plurality of common electrodes 24 diverge from the common line 25 and are formed in parallel with the data line 15. The plurality of pixel electrodes 17 are connected with a drain electrode of the thin film transistor TFT. Each of the pixel electrodes 17 is provided between the common electrodes 24 in parallel. The capacitor electrode 26 extends from the pixel electrode 17 and overlaps with the common line 25.
The thin film transistor TFT is comprised of a gate electrode 12a diverging from the gate line 12, a gate insulating layer (not shown) formed over an entire surface of the thin film transistor array substrate, a semiconductor layer 14 formed over the gate insulating layer, and source and drain electrodes 15a and 15b at both sides of the semiconductor layer 14. The common line 25 is integrally formed with the common electrode 24. The gate line 12 is integrally formed with the gate electrode. Also, the common line 25 and the gate line 12 are simultaneously formed of a low-resistance metal material. Any of the common electrodes 24 may be overlapped with the data line to function as a black matrix, thereby improving an aperture ratio.
The pixel electrodes 17 are formed of a transparent conductive metal material having great transmittance, for example, indium-tin-oxide (ITO), wherein each of the pixel electrodes 17 alternates with each of the common electrodes 24. Also, the pixel electrode 17 is in contact with the drain electrode of the thin film transistor TFT. The capacitor electrode 26 is integrally formed with the pixel electrode 17 to create a storage capacitor.
In the related art IPS mode LCD device, as shown in FIG. 3, if a voltage of 5V is applied to the common electrode 24 and a voltage of 0V is applied to the pixel electrode 17, an equipotential surface is formed in parallel to the two electrodes at the portions right above the two electrodes but is formed in perpendicular to the two electrodes at the portion between the two electrodes. Accordingly, since an electric field is perpendicular to the equipotential surface, a horizontal electric field is formed between the common electrode 24 and the pixel electrode 17, a vertical electric field is formed on the respective electrodes 24 and 17, and both the horizontal and vertical electric fields are formed in the edge of the electrodes 24 and 17.
Alignment of liquid crystal molecules in the related art IPS mode LCD device is controlled with the electric field. For example, as shown in FIG. 4B, if a sufficient voltage is applied to liquid crystal molecules 31 initially aligned in the same direction as a transmission axis of one polarizing sheet, long axes of the liquid crystal molecules 31 are re-aligned to be in parallel to the electric field. When the dielectric anisotropy of the liquid crystal is negative, short axes of the liquid crystal molecules 31 are re-aligned to be in parallel to the electric field. Specifically, first and second polarizing sheets are formed on outer surfaces of the thin film transistor array substrate and the color filter substrate, and the transmission axes of the first and second polarizing sheets are perpendicular to each other, so as to normally display a black mode. If the voltage is not provided to the LCD panel, as shown in FIG. 4A, the liquid crystal molecules 31 are aligned to display the black state. On the other hand, as shown in FIG. 4B, if the voltage is provided to the LCD panel, the liquid crystal molecules 31 are re-aligned to be in parallel to the electric field, thereby displaying the white state.
The liquid crystal material injected between the first and second substrates may deteriorate when a DC voltage is applied for a long time. In order to prevent such a problem, a polarity of the supplied voltage is cyclically changed, which is commonly referred to as a polarity inversion method. The polarity inversion methods include a frame inversion method, a line inversion method, a column inversion method, and a dot inversion method. The dot inversion method is applied to high-resolution devices (i.e., XGA, SXGA, and UXGA) for obtaining a picture image with high quality. In the dot inversion method, a polarity of a data voltage is differently supplied to all-direction adjacent pixels, and therefore it is possible to minimize flicker by spatial averaging. However, the dot inversion method has been problematic since the dot inversion method has a high consumption because of the use of a high-voltage source driver.
A related art IPS mode LCD device of the dot inversion method is described in reference to FIGS. 5 and 6. FIG. 5 illustrates an equivalent circuit view of the related art IPS mode LCD device. FIG. 6 illustrates a timing view of a pixel voltage in each gate line of FIG. 5. As shown in FIG. 5, in a unit pixel of the related art IPS mode LCD device, a thin film transistor TFT is formed adjacent to at each crossing of gate lines (G1, G2, G3, . . . ) and data lines (D1, D2, D3, . . . ). Also, a storage capacitor Cst and a liquid crystal capacitor CLC connected with a drain electrode in each thin film transistor and a common line (Vcom1, Vcom2, Vcom3, . . . ) are formed in parallel between a pixel electrode (‘17’ of FIG. 2).
As shown in FIG. 6, the common voltage Vcom is maintained at a DC voltage having a constant level, even though the signal voltage of the pixel or the gate line is changed or the frame is changed. In this state, the common voltage Vcom is at the intermediate level between two level voltages applied to the data lines. The polarity of the voltage applied to the data line is inversely applied to the respective pixels in each horizontal period. That is, the data voltage is applied such that positive (+) and negative (−) polarities for the Vcom are inversely applied to the respective pixels by alternately applying the positive (+) and negative (−) polarity data voltages to the data lines. At this time, the same polarity of the data voltage is applied to respective odd data lines or respective even data lines.
By applying the gate pulse to the gate line, the thin film transistor of the corresponding line is turned-on. Thus, the video signal applied to each data line through the turned-on thin film transistor is supplied to each pixel. Then, the liquid crystal capacitor CLC and the storage capacitor Cst connected between the drain electrode of the thin film transistor and the common line are charged during a period of the thin film transistor being turned-on. After the thin film transistor is turned-off, electric charges are maintained until the thin film transistor is turned-on.
Referring to FIG. 6, a pixel voltage is changed by a difference amount ΔVp according to a parasitic capacitor Cgs formed between the gate and source electrodes of the thin film transistor along a falling edge of the scanning signal supplied to the gate line, whereby an alignment direction of the liquid crystal material is induced by the difference amount ΔVp. However, when the related art IPS mode LCD device of the dot inversion method is driven, a constant value is supplied to the common voltage signal in the D.C. state, and the positive (+) and negative (−) polarity data voltages for the common voltage signal are alternately supplied to the data lines of the respective pixels. Accordingly, the pixel voltage Vp supplied to the liquid crystal has the polarity dependent on the data voltage, so that it is required to use a source driver having a great output voltage difference to induce a high voltage to the liquid crystal material.
In the related art IPS mode LCD device, the liquid crystal is driven according to a fringe field formed between the pixel electrode and the common electrode. Accordingly, it is required to form the fringe field having a great value by narrowing an interval between the pixel electrode and the common electrode. To narrow the interval between the pixel electrode and the common electrode, it is necessary to pattern the pixel and common electrodes in a finger type crossed at a predetermined interval when the pixel and common electrodes are patterned. However, if the interval between the pixel electrode and the common electrode becomes narrow, an aperture ratio of the pixel is degraded. To improve the aperture ratio, the pixel or common electrode may be formed of a transparent material, such as ITO (Indium-Tin-Oxide). However, since patterns having various shapes are formed within the pixel region, it is difficult to uniformly transmit the light. When the interval between the pixel electrode and the common electrode for improving the aperture ratio is widened, the electric field parallel to the substrates decreases between the pixel electrode and the common electrode. Thus, a high output range of the data voltage must be extended to obtain a required luminance.
Recently, an IPS mode LCD device and a method for driving the same have been proposed to obtain a high liquid crystal voltage between the common electrode and the pixel electrode without using a high output source driver and to improve picture quality with swing of the common voltage by supplying the data voltage and the common voltage of the opposite polarity to the odd/even numbered common lines for the increase of electrode interval and the decrease of driving voltage. FIG. 7 illustrates an equivalent circuit view of a related art IPS mode LCD device for increasing the electrode interval and decreasing the driving voltage. FIG. 8 illustrates a timing view of a pixel voltage in each gate line of FIG. 7.
As shown in FIG. 7, a plurality of gate lines (G1, G2, G3, G4, . . . ) are perpendicular to a plurality of data lines (D1, D2, D3, D4, . . . ). Also, each common line (Vcom1, Vcom2, Vcom3, . . . ) is formed between the gate lines, and a thin film transistor TFT is formed adjacent to a crossing of the gate and data lines. Further, a first storage capacitor Cst and a first liquid crystal capacitor CLC connected with a drain electrode of the thin film transistor are formed in parallel, between the common line and a pixel electrode (‘17’ of FIG. 2).
For the increase of the electrode interval and the decrease of the driving voltage in the related art IPS mode LCD device, when a first common voltage (or second common voltage) is applied to the odd numbered common line (Vcom1, Vcom3, . . . ), a second common voltage (or first common voltage) is applied to the even numbered common line (Vcom2, Vcom4, . . . ). In this state, the data voltage of the same polarity is applied to the pixels connected with the same common line. That is, as shown in FIG. 8, if the data voltage of positive (+) polarity is applied to a predetermined pixel, the first common voltage (Vcom(−)) is applied to the corresponding common line. On the other hand, if the data voltage of negative (−) polarity is applied to a predetermined pixel, the second common voltage (Vcom(+)) is applied to the corresponding common line. Accordingly, a voltage difference increases between the pixel electrode and the common electrode. This related art IPS mode LCD device has a common voltage driving circuit to divide the common lines into odd numbered common lines and even numbered common lines and to apply the common voltage to the odd/even numbered common lines separately.
FIG. 9 illustrates a circuit diagram of the common voltage driving circuit by a common voltage swing method according to the related art. FIG. 10 illustrates a timing view of an output waveform of FIG. 9. As shown in FIG. 9, the related art common voltage driving circuit includes a first common voltage output part 50 for swing and outputting the common voltage of positive (+) and negative (−) polarity to the odd numbered common lines, and a second common voltage output part 60 for swing and outputting the common voltage of positive (+) and negative (−) polarity to the even numbered common lines. Herein, the first and second common voltage output parts 50 and 60 respectively include first and second dividers 51 and 61, first and second inversion amplifiers 52 and 62, and first and second push/pull amplifiers 53 and 63. The first divider 51 comprising resistance Ru1 and Rv and the second divider 61 comprising resistance Ru2 divide a constant voltage VLCD. The first and second inversion amplifiers 52 and 62 amplify and output respective voltages output from the first and second dividers 51 and 61 according to first and second control signals CNT1 and CNT2 output from a timing controller (‘3’ of FIG. 1). Then, the first and second push/pull amplifiers 53 and 63 re-amplify the respective voltages output from the first and second inversion amplifiers 52 and 62, and output the re-amplified voltages to the odd numbered common lines and the even numbered common lines.
Next, an output of the related art common voltage driving circuit is described with reference to FIG. 10. As shown in FIG. 10, the first and second control signals CNT1 and CNT2 having the opposite phases are output from the timing controller. Thus, the first common voltage output part 50 and the second common voltage output part 60 swing the common voltages to have the opposite polarity by using the first and second inversion amplifiers 52 and 62 and the first and second push/pull amplifiers 53 and 63. That is, the first and second inversion amplifiers 52 and 62 compare the respective voltages divided by the first and second dividers 51 and 61 with the first and second control signals CNT1 and CNT2 output from the timing controller, and then amplify and output the respective voltages. The first and second push/pull amplifiers 53 and 63 amplify the voltages output from the first and second inversion amplifiers 52 and 62 to signals having great linearization and less distortion, and then output the amplified voltages.
The related art common voltage driving circuit has the following disadvantages. The related art common voltage driving circuit swings the common voltages with the inversion amplifier, whereby an A.C. consumption voltage (PAC) of the IPS mode LCD device is obtained as follows,PAC=n×C×f×(VCH−VCL)2,wherein, ‘n’ is the number of common voltages swung, ‘C’ is a capacitor load of the common voltage, a total amount of a storage capacitor amount and a parasitic capacitor amount between the common line and the data line, ‘f’ is a frequency of the common voltage, and (VCH−VCL) is a swing width of the common voltage. Accordingly, the common voltage swing driving circuit of the related art IPS mode LCD device utilizes the inversion amplifier, so that the common voltages are repetitively swung between the highest value ((+) common voltage) and the lowest value ((−) common voltage), thereby increasing the power consumption.